SingleAssignmentLanguage

From Epowiki

A language in which a variable can only be assigned once. A single assignment language is a functional language, for an expression like x := sin(pi / 6) can be thought of as defining a nilary function x which returns the value of the above expression.

See also Single-Assignment C (http://www.informatik.uni-kiel.de/sacbase/), and SISAL (http://www.llnl.gov/sisal/).